GT750 regulator

All to do with wiring, charging or just trying to figure out whats gone wrong.

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titan performance
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GT750 regulator

Post by titan performance »

Calling all 750 electrics experts. To be honest, I'm a bit of a dunce with electrics, but I'm sure this is an easy one.
My 750 runs lovely, but gradually flattens the battery. I guess there is a charging issue and my first thought are....regulator? It's a 1973 bike, and I have posted a pic of the reg. The battery is low having been laid up all winter, but it started easy enough with a starter pack. However, voltage at the battery remains steadyish in the high elevens when running, and doesn't increase when revved.
These regs are adjustable aren't they? Anyone point me in the right direction please?

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Re: GT750 regulator

Post by titan performance »

titan performance wrote:Calling all 750 electrics experts. To be honest, I'm a bit of a dunce with electrics, but I'm sure this is an easy one.
My 750 runs lovely, but gradually flattens the battery. I guess there is a charging issue and my first thought are....regulator? It's a 1973 bike, and I have posted a pic of the reg. The battery is low having been laid up all winter, but it started easy enough with a starter pack. However, voltage at the battery remains steadyish at a bit under 12v even when the engine is revved. Anyone point me in the right direction please? Aren't these regs adjustable?



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Re: GT750 regulator

Post by titan performance »

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Re: GT750 regulator

Post by jabcb »

You could have more than one problem.

Start with the battery, which could be shot. Take it out of the bike & charge it up. Then leave it sit overnight & check the voltage the next morning.

What type of battery do you have? Do you have a battery charger/maintainer suitable for your battery?
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Re: GT750 regulator

Post by titan performance »

Thanks for the response Jab.....I had this problem when I used the bike at last year's TT. The battery was good then, but still it gradually discharged. The battery is in the tail unit, so I have used a smaller one as space was at a premium, but when charged, it spun the motor over fine. I think it's a 12N7 4A from a 500.
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Re: GT750 regulator

Post by jabcb »

If the problem is the voltage regulator, then you can adjust it or replace it with a modern aftermarket part.

Here in the US, http://www.oregonmotorcycleparts.com has a good adjustable voltage regulator for about $50.
Its a direct replacement that uses the same mounting.
You may find that you have to adjust the aftermarket voltage regulator because of the voltage drops in your wiring.
Perhaps one of the UK members can suggest an alternative available on your side of the pond.

The stock voltage regulator is a simple electromagnet working against a spring. To increase the voltage, you simply stiffen the spring a smidgen. I think the GT750 shop manual shows how to do this.


The problem could be something else, like worn out or cracked brushes.


A 12N7-4A would be a conventional battery that you have to add water to.
My guess would be that you likely have an equivalent maintenance free battery.
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Re: GT750 regulator

Post by pearljam724 »

Simply hook up a multimeter to the battery to use as a gauge. You have to read the multimeter's reading during idle and mid rpm ranges. Regulators are extremely sensitive to the smallest adjustment you would make with the points. No big deal, all that means is. You will likely have to readjust them several times until you get the charge you are looking for on the multimeter because the are so sensitive. A millimeter or 2 is the difference between too much or too little in difference of over or undercharging. Very simple to do, though. A manual will help by knowing the recommended gap tolerance between the point and contact. But, it's not necessary if you simply watch the multimeter knowing what the battery should be charging at at idle and mid Rpms. Roughly, 12.8 at idle and 14.2 volts at mid rpm. I would bet on the battery having a dead cell before the regulator needing adjusted. Test the battery properly first. Regulators points wear, just like points on the ignition system. So, it could possibly need looked at as well. You adjust the point gap by turning that little silver screw on the left of your picture. That screw. Increases or decreases point gap. Turning it clockwise will decrease gap. Vise versa. Turn the screw a hair at a time as you verify the changes on the multimeter. Extremely sensitive. A bad rectifier will fry a battery due to extreme overcharging. You can test that with a multimeter also. Age of battery or it being hooked up to a battery maintainer means very little to the condition of the battery. You have to properly test it, under a few conditions.
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Re: GT750 regulator

Post by titan performance »

Thank you guys.....very helpful.
I did hook my multi meter to the battery while the engine was running, but the volts were constant-ish, within a few points, at under 12....I'll have another try and make some tiny adjustment on the regulator to see if that changes the voltage at the battery at all. If it doesn't, I guess I have to assume either the reg or my alternator are at fault, or maybe a wiring issue.
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Re: GT750 regulator

Post by Alan H »

Next thing will be to check the carbon brushes on the rotor.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
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Re: GT750 regulator

Post by pearljam724 »

Battery is no good, dead cell. If it's reading under 12 volts at idle. That would be the case if it was fully charged prior to you testing it. A battery with a dead cell will fully charge and hold that charge for a few days. But, as soon as voltage is drawn the battery voltage nose dives. You can test that also. As soon as you completely charge it. Let the battery set idle for a few days checking voltage every 12 hours or so. You'll see the voltage ever so slightly decrease over those few days if there is a dead cell. Which batteries straight off a store shelf can have or develope in a short time frame. The best test for a dead cell is watching the multimeter when you attempt to fire the motor via the electric starter. If voltage dips below 8 or 9 volts. The battery has a dead cell if it was fully charged prior.
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Re: GT750 regulator

Post by titan performance »

Many thanks guys......as luck would have it, a slight tweak on the adjustment screw, and up went the voltage at the battery.
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